A favorable system which is amenable to frequent and reproducible sampling, consisting of suspension cultures of strain L cells and vaccinia virus, was employed to study the animal virus-mammalian host cell relationship. The three principal aspects investigated concerned the adsorption and penetration of vaccinia into the host, the relationship between the sequence of virus development and the production of infectious particles, and the changes in the fine structure of the host cells. Experiments in which a very high multiplicity of infection was used revealed that vaccinia is phagocytized by L cells in less than 1 hour after being added to the culture, without any apparent loss of its outer limiting membranes. Regions of dense fibrous material, thought to be foci of presumptive virus multiplication, appear in the cytoplasm 2 hours after infection. A correlation between electron microscope studies and formation of infectious particles shows that although immature forms of the virus appear 4 hours after infection, infectious particles are produced 6 hours after infection of the culture, at the time when mature forms of vaccinia appear for the first time in thinly sectioned cells. Spread of the infection is gradual until eventually, after 24 hours, virus is being elaborated throughout the cytoplasm. Addition of vaccinia to monolayer cultures induced fusion of L cells and rapid formation of multinucleate giant forms. In both suspension and stationary cultures infected cells elaborate a variety of membranous structures not present in normal L cells. These take the form of tube-like lamellar and vesicular formations, or appear as complex reticular networks or as multi-laminar membranes within degenerating mitochondria.